1. Field of the Invention
This invention relates to large-mode-area (LMA) fibers designed to compensate for the effects of bends in the fiber and to suppress higher-order modes (HOMs) and, more particularly to high power optical amplifiers that incorporate such LMA fibers.
2. Discussion of the Related Art
Optical fiber amplifiers have great impact in diverse applications ranging from high power devices used for cutting, welding and range finding to lower power devices used to amplify optical carrier signals in telecommunication systems. In the former case, the high power amplifier utilizes a gain-producing fiber (GPF; e.g., a LMA fiber doped with suitable rare-earth species or chromium) and a source of pump light to amplify signal light propagating in effectively a single mode (i.e., the fundamental transverse mode) through the LMA fiber.
LMA fibers, which have a relatively large effective area (Aeff), are used to reduce optical power density and, therefore, also reduce optical nonlinearities in the fiber. However, larger area fibers typically support several or many modes, increasing the likelihood that HOMs will also propagate in the fiber and undergo amplification, thereby degrading the quality of beam. Beam quality is often characterized in terms of a parameter known as M2 (M2=1 for an ideal Gaussian beam), whereas single modedness can be characterized by various techniques including spatially and spectrally (S2) resolved imaging, as described by Nicholson et al., Optics Express, Vo. 16, No. 10, pp. 7233-7243 (2008), which is incorporated herein by reference. Bends in the fiber exacerbate this problem—they reduce the ability of various fiber designs to selectively suppress HOMs while ensuring propagation of the fundamental mode at power levels that satisfy typical performance requirements.
In a typical conventional amplifier configuration a few meters (e.g., 5 m) of GPF is coiled within an amplifier package that may also contain other components of the amplifier. In some designs, those components include a non-GPF LMA (e.g., a fiber pigtail) optically coupled to the GPF. Coiling the LMA fiber, an expedient to save space, means that the fiber is bent.
Bends in the LMA fiber are a key factor imposing performance tradeoffs between three principal goals of LMA fiber design: large mode area, low loss, and single-mode operation. Macrobend loss is often the dominant source of loss, bend distortion limits the scaling of area, and bends degrade single-mode operation, as noted above, by limiting the degree to which unwanted HOMs can be selectively suppressed.
One strategy for ameliorating the adverse effects of bending is to pre-compensate the refractive index profile of an unbent (as-fabricated, straight) fiber for the expected bend-induced perturbation, as described by Fini, Opt. Express, Vol. 14, No. 1, pp. 69-81 (2006), which is incorporated herein by reference. This strategy, which utilizes an asymmetric index profile, has been exploited by others in the design of bend-compensated microstructure fibers. [See, for example, Minelly, U.S. Pat. No. 7,876,495 (2011), which is also incorporated herein by reference.] However, this strategy may be difficult to implement. It requires an asymmetric index profile across the fiber cross-section, and it requires deployment of the fiber in a fixed azimuthal orientation throughout the bend.
By the terms unbent, straight, and as fabricated we mean the bend radius of the fiber is essentially infinite (a perfectly straight fiber) or so large that any resulting bend would have an insignificant effect on the fiber performance for the intended application of the fiber.
Thus, there is a need for an LMA fiber design that provides bend compensation without requiring an asymmetric index profile. That is, there is a need for a fiber design that provides bend compensation in LMA fibers that have either asymmetric or symmetric index profiles.
In addition, there is a need for a LMA fiber design that provides for HOM suppression in addition to compensation.